Nature's Den

Nature's Den is a cave on Route 12, only accessible with HMs Rock Smash and Cut. There are no general wild Pokémon encounters, but the Legendary Trio Forces of Nature reside here.

Catching Forces of Nature
Venturing deep into the cave, a shrine can be seen with sunlight shining onto, with Tornadus and Thundurus in their Incarnate Forms looking at it. Upon reaching the shrine, Tornadus and Thundurus are startled by the player. Instead of battling, they immediately fly high and flee. From this point onward, they can be found as Roaming Pokémon on any location with wild Pokémon encounters.

After the player catches both of them, return to the shrine with both Tornadus and Thundurus in party. The Trio Master Landorus will then be seen waiting for the player. Rather than fleeing, it will instantly battle the player, serving as the only Pokémon battle in this place.

Legendary Pokémon
Players only have 1 chance to battle the following Pokémon. Save beforehand and turn off the Auto Save function. Requires both Tornadus and Thundurus in party.

Trivia

 * This is the only cave with no general wild Pokémon encounter.
 * Scorched Slab, Hoenn once shared this trait in Pokémon Ruby, Sapphire and Emerald, but it was expanded in remakes Pokémon Omega Ruby and Alpha Sapphire with wild Pokémon added in.